Electronic circuit for converting a current signal to a voltage signal

ABSTRACT

The invention relates to a method and to an electronic circuit for converting a current signal Isignal to a voltage signal Vout) comprising:—a first resistor Rgain and a second resistor Rconversion,—means ( 2 ) for generating a first current Igain based on a reference voltage Vreference applied over the first resistor,—means ( 3, 4 ) for generating a second current, the magnitude of the second current being determined on the basis of the multiplied magnitude of the first current and the current signal,—means ( 5 ) for applying the second current to the second resistor for generating the voltage signal.

FIELD OF THE INVENTION

The present invention relates to the field of converting a currentsignal to a voltage signal, and more particularly to the usage of anintegrated resistor for the purposes of the current-voltage conversion.

BACKGROUND AND PRIOR ART

A variety of manufacturing methods for integrated resistors is knownfrom the prior art. It is convenient to make these resistors in the sameprocess steps as the rest of the integrated circuit. Therefore, theexact way in which the resistors are made depends on which process isused to produce the integrated circuit.

Integrated circuits are generally fabricated with polysilicon resistorsthat are formed on the semiconductor substrate. Such resistors generallyhave a resistor body, formed of doped polysilicon and having metallicleads coupled to opposing ends of the resistor body, generally thoughcontacts in field oxide. The contacts are connected to metalinterconnect. The resistor body can be formed concurrently withpolysilicon transistor gate electrodes, such resistor body generallydoped and resting on the field oxide.

Integrated circuits that require passive resistors often have tighttolerances on the resistance value of these resistors. However, theseprior art semiconductor resistors are subjected to variations inresistance value. Sources of variation in the resistance value of thesesresistors include process fluctuations that result in physical changesto the resistor properties such as physical dimension or resistivity andchanges in temperature. The sources of change in temperature can beeither external to the resistor itself or internal due to theself-heating effects associated with power dissipation. As the resistortemperature changes, the value of resistance of the resistor alsochanges.

The general prior art method utilized for minimizing the resistancealteration effects due to the temperature coefficient of resistance(TCR) of semiconducting resistor (a resistor formed of semiconductormaterial) is to increase the doping concentration in the resistor bodyto a sufficiently high level such that the TCR of the resistor body isat a minimum. Then the resistors are built with dimensions that make theresistor head resistance a small percentage of the resistor bodyresistance. As a result, the resistor heads TCR has little effect on theoverall resistor temperature characteristics.

A corresponding integrated circuit with an integrated resistance havinga minimized temperature coefficient is shown in U.S. Pat. No. 6,333,238.Another method for producing a semiconductor integrated circuit with anintegrated resistor is known from U.S. Pat. No. 6,329,262.

Further it is known to use integrated resistors for digital to analogueconverters. U.S. Pat. No. 5,604,501 shows a digital-to-analog converterincluding a resistor string having intermediate taps at resistorjunctions as well as resistor-potential junctions. Switching transistorsare coupled between a respective intermediate tap and an output node.Decode circuits are capable of switching at least two transistors to bein the on state at the same time to electrically couple more than oneintermediate tap to the output node to produce at least one analogoutput. One row select line can be energized simultaneously with atleast two column select lines. Alternatively, at least two row selectlines can be energized simultaneously with one column select line.

U.S. Pat. No. 6,278,393 shows an integrated circuit including adigital-to-analog converter in which a resistor string is adapted to becoupled to a reference source. The resistor string includes a pluralityof serially coupled impedances defining intermediate taps at thejunctions thereof. A first plurality of switches are coupled between afirst output node and respective ones of the intermediate taps.

A first selection circuit receives a first digitally codes signal and iscoupled to each switch in the first plurality of switches. The firstselection circuit selectively switches the first plurality of switchesto predetermined states depending upon a first digitally coded signalprovided thereto, to generate a first analog output. A second pluralityof switches are coupled between a second output node and respective onesof the intermediate taps. A second selection circuit coupled to eachswitch in the second plurality of switches selectively switches thesecond plurality of switches to predetermined states depending upon asecond digitally codes signal provided thereto, to generate a secondanalog output.

Generally the resistance tolerance of an integrated resistor is withinabout ±20%. Integrated resistors having a high square resistance haveeven higher tolerances. The lack of a method to produce integratedresistors having a precise resistance value has limited the field of useof integrated resistors in the prior art.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedelectronic circuit for converting a current signal to a voltage signalby means of a resistor with a high degree of precision and which issuitable for monolithic integration.

The present invention provides an electronic circuit for converting acurrent signal to a voltage signal comprising:

-   -   a first resistor and a second resistor,    -   means for generating a first current based on a reference        voltage applied over the first resistor,    -   means for generating a second current, the magnitude of the        second current being determined on the basis of the multiplied        magnitudes of the first current and the current signal,    -   means for applying the second current to the second resistor for        generating the voltage signal.

Further embodiments of the electronic circuit of the invention are givenin the dependent claims.

Further the invention provides for a method for converting a currentsignal to a voltage signal comprising the steps of:

-   -   generating a first current by means of applying a reference        voltage to a first resistor,    -   generating a second current, the magnitude of the second current        being determined on the basis of the multiplied magnitudes of        the first current and the current signal,    -   applying the second current to the second resistor for        generating the voltage signal.

Further embodiments of the method of the invention are given in thedependent claims.

The invention is particularly advantageous as it allows to convert acurrent signal to a voltage signal by means of first and secondresistors whereby the conversion is performed with a high degree ofprecision. To obtain a high degree of precision it is not necessary toprovide precision resistors having exact resistance values.

Rather the precision of the conversion in accordance with the presentinvention is determined by the precision of the ratio of the resistancevalues of the first and second resistors. This makes the electroniccircuit of the present invention particularly suitable for monolithicintegration as it is possible to produce a pair of integrated resistorshaving a precise resistance ration without difficulty.

One field of application of the present invention is for CMOS integratedcircuits, such as processors with analogue current outputs. Typicallysuch an analogue current output needs to be converted to a voltage forexample for the purposes of amplification. The problem of providing aprecise and cost efficient interface between the processor and thefollowing amplification stage is solved by means of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a first embodiment of an electroniccircuit in accordance with the invention,

FIG. 2 shows a block diagram of a second embodiment of an electroniccircuit in accordance with the present invention,

FIG. 3 shows a block diagram for the generation of the circuit Igain inthe embodiments of FIGS. 1 and 2,

FIG. 4 shows a detailed embodiment of the electronic circuit of FIG. 1,

FIG. 5 shows a detailed embodiment of the electronic circuit of FIG. 2,

FIG. 6 to 9 show a variety of detailed embodiments for the voltageconversion in the embodiments of FIGS. 1 and 2.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of an electronic circuit 1 for converting acurrent signal Isignal to a voltage signal Vout. The electronic circuit1 has two integrated resistors Rgain and Rconversion. A referencevoltage Vreference is provided to the electronic circuit 1 or generatedwithin the electronic circuit 1. By means of the reference voltageVreference and the integrated resistor Rgain a current Igain isgenerated in block 2 of the electronic circuit 1. The current Igainequals Vreference divided by Rgain.

The current Igain is provided to a divider 3. Further a referencecurrent Ireference is provided to the divider 3. The purpose of theprovider 3 is to divide the current Igain by the reference currentIreference. The resulting current Igain/Ireference is provided tomultiplier 4. Further the current signal Isignal to be converted isapplied to multiplier 4. The multiplier 4 provides an output currenthaving a magnitude which is determined on the basis of the magnitudes ofthe current signal Isignal and the current Igain divided by thereference current Ireference. In the example considered here the outputcurrent provided by the multiplier 4 is the product of Isignal and Igaindivided by Ireference.

The output current of the multiplier 4 is provided to block 5 of theelectronic circuit 1 which contains the integrated resistor Rconversion.The block 5 serves to convert the output current of the multiplier 4 tothe voltage signal Vout. This is done by applying at least a portion ofthe output current of the multiplier 4 to the integrated resistorRconversion. As a consequence the voltage signal Vout is as follows:${Vout} = {\frac{Isignal}{Ireference}{Vreference}\frac{Rconversion}{Rgain}N}$Where N is a constant and depends on the configuration of theimplementation. Convenient choices for N are N=½ or N=1.

As it appears from the above formula the value of the voltage signalVout does not depend on the absolute values of Rgain and Rconversion buton the ratio of Rconversion and Rgain. Hence, even though the absolutevalues of the resistances of Rgain and Rconversion may have largetolerances, the ratio of the resistances of Rgain and Rconversion can bemanufactured with a high degree of precision such that the current tovoltage conversion of the present invention is also accomplished with ahigh degree of precision even though integrated resistors are utilized.

FIG. 2 shows an alternative embodiment of the electronic circuit 1 wherelike elements are denoted by the same reference numerals as in theembodiment of FIG. 1.

In the embodiment of FIG. 2 a current signal Isignal is applied to thedivider 3 as well as the reference current Ireference. The outputprovided by the divider 3 is Isignal/Ireference. This value is inputtedinto the multiplier 4.

Further the multiplier 4 receives the current Igain from the block 2.This current Igain is multiplied by Isignal/Ireference in the multiplier4. The mutliplier 4 outputs the product of Igain and Isignal divided byIreference.

The output of the multiplier 4 is provided to the block 5 for current tovoltage conversion in the same way as in the embodiment of FIG. 1.

FIG. 3 shows a detailed embodiment of the block 2 of the embodiments ofFIGS. 1 and 2. The reference voltage Vreference is either externallyapplied to the electronic circuit 1 or generated within electroniccircuit 1. The reference voltage Vreference is applied over theintegrated resistor Rgain. One of the terminals of the resistor Rgain isconnected to current mirror 6 which is connected to ground. This way thecurrent Igain is obtained.

FIG. 4 shows a more detailed embodiment of the electronic circuit ofFIG. 1. The current signal Isignal is split by means of the differentialpair of bipolar transistors Q3 and Q4. A differential base-emittervoltage is provided to the differential pair of bipolar transistors Q3and Q4 from the circuit arrangement comprising transistors Q1, Q2, Q5and Q6.

The bases of the transistors Q1 and Q2 are coupled to a common potentialwhich is also common to the collectors of Q1 and Q2. The referencecurrent Ireference is applied to this common potential. The emitter ofthe transistor Q1 is coupled to the collector of the transistor Q5. Theemitter of the transistor Q5 is coupled to Rgain.

The emitter of the transistor Q2 is coupled to the collector of thetransistor Q6. The emitter of the transistor Q6 is coupled to a furtherreference voltage Vreference.

The base of the transistor Q6 is coupled to the emitter of thetransistor Q1; the base of the transistor Q5 is coupled to the emitterof the transistor Q2. The differential base-emitter voltage for thedifferential pair of bipolar transistors Q3 and Q4 is provided betweenthe potential of the emitter of transistor Q2 and the emitter oftransistor Q1.

FIG. 5 shows an embodiment which corresponds to the embodiment of FIG.2. In comparison to the embodiment FIG. 4 the currents Isignal and Igainare interchanged. As a result the current Igain is split rather than thecurrent Isignal. The voltage Vin determines the input voltage at theemitter of Q5. The resulting current in the collector branches of thetransistors Q3 and Q4 is the same as in the embodiment of FIG. 4.

FIG. 6 shows one way of converting the compensated current signal tovoltage by means of the integrated resistor Rconversion. The collectorbranches of the transistors Q3 and Q4 (cf. FIG. 4 and FIG. 5) areconnected to current mirror 7. The resistor Rconversion is connectedbetween the collector potential of the transistor Q4 and VDD to producethe voltage Vout.

FIG. 7 shows an embodiment of block 5 of FIGS. 1 and 2 with feedback. Toprovide feedback an operational amplifier 8 is used for producing thevoltage Vout. A voltage reference Vx is connected to the non-invertinginput of the operational amplifier 8. The inverting input of theoperational amplifier 8 is coupled to the collector potential of thetransistor Q4. The resistor Rconversion is coupled between the invertinginput of the operational amplifier 8 and the output of the operationalamplifier 8.

FIG. 8 shows an alternative embodiment where only one of the collectorcurrents is used for the conversion. For this purpose the resistorRconversion is coupled between VDD and the collector potential of thetransistor Q4 in order to produce the output voltage Vout. Thiscorresponds to a choice of N=¹².

FIG. 9 shows a further embodiment for producing a differential outputvoltage Vout1 and Vout2. A resistor Rconversion is coupled between VDDand collector potential for both transistors Q3 and Q4. The differentialvoltage Vout=Vout1−Vout2 can be used for input to an amplifier in acomputer monitor or for similar applications requiring a differentialoutput voltage.

In essence it is possible to implement the present invention by means ofan extra line per input current signal and a separate line to providethe reference current Ireference. For an audio-mono signal this meansthat two lines are required—one for the current signal Isignal and onefor the reference current Ireference. For an audio-stereo signal anadditional line for the additional audio channel is required. For thecase of RGB-video three signal lines for the three channels are requiredin addition to the line for the reference current Ireference.

Rather than inputting the signal current(s) and the reference current onseparate lines a differential input can be provided. In the case of anaudio-mono signal this can be implemented by providing a signalIreference−Isignal on one line and Ireference+Isignal on the other line.Likewise this can be implemented for more than one channel.

Alternatively the number of input lines can be restricted to the numberof channels. In other words a separate line for Ireference can beavoided. This can be accomplished by utilizing the current signal itselfas a basis to form the reference current Ireference. This can be done byusing a maximum, a minimum, an average value or the RMS of Isignal as areference. For example, in the case of a video signal the black signalof each video frame or line can be utilized as the reference Ireference.

In case of more than one channel one or more or all of the channels canbe used to form the reference current Ireference.

As a further alternative the input line for the signal Isignal is timemultiplexed to provide the reference current Ireference. In case of morethan one channel one or more or all of the channels can be utilized fortime multiplexing the reference current.

LIST OF REFERENCE NUMERALS

-   electronic circuit 1-   block 2-   divider 3-   multiplier 4-   block 5-   current mirror 6-   current mirror 7-   operational amplifier 8

1. An electronic circuit for converting a current signal (Isignal) to avoltage signal (Vout) comprising: a first resistor (Rgain) and a secondresistor (Rconversion), means (2) for generating a first current (Igain)based on a reference voltage (Vreference) applied over the firstresistor, means (3, 4) for generating a second current, the magnitude ofthe second current being determined on the basis of the multipliedmagnitude of the first current and the current signal, means (5) forapplying the second current to the second resistor for generating thevoltage signal.
 2. The electronic circuit of claim 1 the means forgenerating comprising: means (3) for dividing the first current by areference current (Ireference) to determine a first parameter(Igain/Ireference), means (4) for multiplying the current signal and thefirst parameter to generate the second current.
 3. The electroniccircuit of claim 1 the means for generating comprising: means (3) fordividing the current signal by a reference current to determine a firstparameter (Isignal/Ireference), means (4) for multiplying the firstcurrent and the first parameter to generate the second current.
 4. Theelectronic circuit of claim 1, the means for generating the firstcurrent comprising current mirror means (6).
 5. The electronic circuitin accordance with claim 1 the means for generating the second currentcomprising: a differential pair of bipolar transistors (Q3, Q4), means(Q1, Q2, Q5, Q6) for applying a differential base-emitter voltage to thedifferential pair of bipolar transistors, the differential voltage beinga logarithmic function of the first current divided by a referencecurrent.
 6. The electronic circuit of claim 5, only one of the collectorcurrents of the differential pair of bipolar transistors being coupledto the second resistor.
 7. The electronic circuit of claim 5, bothcollector currents of the differential pair of bipolar transistors beingcoupled to the respective ones of second resistors to produce adifferential voltage signal.
 8. The electronic circuit of claim 1comprising means for providing the reference current based on a maximumcurrent signal or a minimum current signal or an average current signalor the RMS of the current signal.
 9. The electronic circuit of claim 8the means for providing the reference current being adapted to providethe reference current based on the black signal component of a currentvideo signal.
 10. The electronic circuit of claim 1 further comprisingmeans for time multiplexing of an input line for the current signal inorder to provide a time multiplexed reference current.
 11. Theelectronic circuit of claim 1, the electronic circuit being anintegrated circuit and the first and second resistors being integratedin the integrated circuit.
 12. A method for converting a current signalto a voltage signal comprising the steps of: generating a first currentby means of applying a reference voltage to a first resistor, generatinga second current, the magnitude of the second current being determinedon the basis of the multiplied magnitudes of the first current and thecurrent signal, applying the second current to the second resistor forgenerating the voltage signal.
 13. The method of claim 12 the step ofgenerating a second current comprising the steps of: dividing the firstcurrent by a reference current to determine a first parameter,multiplying the current signal and the first parameter to generate thesecond current.
 14. The method of claim 12 the step for generating thesecond current further comprising: dividing the current signal by areference current to determine a first parameter, multiplying the firstcurrent and the second parameter to generate the second current.
 15. Themethod of claim 12 further comprising providing the reference currentbased on a maximum of the current signal, a minimum of the currentsignal, an average of the current signal or a RMS value of the currentsignal.
 16. The method of claim 12 further comprising providing thereference current by time multiplexing an input line for the currentsignal.